Discharge lamps of different forms have been in use for about a century. Today, gas discharge lamps, such as mercury vapor, sodium vapor and metal halide lamps, continue to represent a substantial fraction of the lighting industry. Typically, the lamps are formed from a sealed vessel which holds the vapor or gas, and is electrically excited by a voltage applied between metal electrodes. However, conventional lamps suffer from several drawbacks, one of which is the maximum operating gas (or vapor) pressure. For some lamps such as arc lamps, the pressure is limited by the strength of the vessel material, which must be transparent or translucent to create an effective light source. Others, such as hollow cathode lamps, have a maximum gas pressure at which hollow cathode discharge operation can be achieved. Generally fabricated in metals, hollow cathodes for conventional discharge lamps typically have diameters on the order of millimeters or centimeters and are normally limited to operation at pressures of a few Torr.
One approach to addressing these limitations for high pressure arc lamps is proposed in U.S. Pat. No. 5,438,343 to Khan et al. which contemplates a large number of microcavities, each of which can operate at a higher pressure than a single large cavity. The microcavities are formed by wafer bonding of two micromachined substrates of fused quartz, sapphire, glass or other transparent or translucent material. Cavities in the separate substrates align to form vessels for containing a gas or other "filler" (e.g., metal, metal-halide, etc.) after the substrates are bonded. While a RF "electrodeless" embodiment is disclosed, other embodiments include etched recesses adjacent to the vessels in one or both of the substrates for accommodating separate metal electrodes. After the electrodes are deposited or otherwise placed in the recesses to electrically contact the discharge medium, the separate substrates are bonded together by van der Waal's forces.
Separate plugs are required at the point where the electrode connections enter the vessel wall to maintain the vacuum integrity of the device. The plug material, which may be glass, is deposited over metal electrodes to reinforce the microcavity which is weakened by the recess necessary to accommodate a separate electrode. Together, the reliance on van der Waal's forces to bond separate substrates and the need for reinforcing plugs significantly complicate the production of the device. Another difficulty with the lamp devised by Khan et al. concerns the substrate material itself. Sapphire, fused quartz and other materials used in U.S. Pat. No. 5,438,343 for transparent or translucent substrates are brittle and difficult to process. The operation of the Khan device is also limited to a positive column discharge by the device geometry.
Others have proposed cavities in hollow metal cathodes having diameters as small as approximately 1 mm. As early as 1959, White, "New Hollow Cathode Glow Discharge," J. Appl. Phys. 30, 711 (1959), examined hollow cathode devices having typical diameters of 750 .mu.m formed in a variety of metals, including molybdenum and niobium. More recently, Schoenbach et al., "Microhollow Cathode Discharges," Appl. Phys. Lett. 68, 13 (1996), produced and studied hollow cathode lamps having cavities with diameters of approximately 700 .mu.m machined in molybdenum and insulators made of mica. However, the processes used to produce cavities having diameters of approximately 700 .mu.m in bulk metals are not conducive to mass production or to the fabrication of arrays of microdischarges. In addition, sputtering of the metal cathode limits device lifetime.
Schoenbach et al. also recognized the benefit of cavities smaller than 700 .mu.m. Although Schoenbach et al. reported an effective cavity of 75 .mu.m in molybdenum, this structure consisted of a machined hole having a diameter on the order of 700 .mu.m forming most of the cathode, and a smaller 75 .mu.m cathode opening, thus producing a microcavity aperture only at the top of the device. This arrangement would not lend itself to the mass production of inexpensive devices, and it is not clear that the performance characteristics of such a two-section cathode would be similar to a true microcavity cathode having a maximum diameter from below about 500 .mu.m down to about a single micrometer. Another concern with metal cathode devices is the formation of metal-bearing compounds (including the metal halides) that are a byproduct of the reaction of various metals with some discharge media that are useful, such as the halogens.
These issues have important implications for a variety of microdischarge applications, and their potential as displays and lighting sources, in particular. The leading candidates currently being pursued for high resolution displays are liquid crystals, field emission devices, and plasma panels. Large area displays have largely been the domain of plasma panels which are now available in 42" diagonal displays. However, plasma panels present formidable manufacturing challenges stemming from the materials employed and the approach that has been adopted for producing the display. Discharge gaps, typically 100 to 300 .mu.m in commercial devices, are defined by the spacing between metal electrodes, one of which is often a wire (see, for example, Kyung Cheol Choi, "Microdischarge in microbridge plasma display with holes in the cathode," IEEE Electron Dev. Lett. 19, 186 (1998)). Precisely constructing scores (or thousands) of microdischarge devices so that the discharge gap does not vary significantly among the discharges is a difficult task.
The other display technologies currently under consideration also suffer from several drawbacks. Despite their use in portable and desktop computer displays, for example, liquid crystals are limited in brightness and offer a restricted viewing angle. Field emission devices rely on processing silicon pyramidal structures by VLSI fabrication techniques. These devices produce a weak current when a voltage is applied between the tip of the Si pyramid (or cone) and an electrode (anode). The magnitude of the emission current is sensitive to the gap between the two which, combined with the requirement that the device operate in a vacuum, mandates sophisticated manufacturing processes and has thus far limited the sizes of field emission displays to typically 5-10" (along the diagonal).
Accordingly, it is an object of the present invention to provide an improved microdischarge device that eliminates several limitations associated with the manufacture and performance of conventional lamps and displays.
A further object of the present invention is to provide an improved microdischarge device having at least one microcavity electrically contacted to a one-piece or multilayered substrate which forms a cathode for the microcavity.
Another object of the present invention is to provide an improved microdischarge device including a microcavity in a silicon substrate which contains a conductive medium ("filler"), such as gas or vapor, wherein the filler is electrically contacted by a semiconductor cathode formed in the silicon around the microcavity.
An additional object of the present invention is to provide an improved microdischarge device and array design including a microcavity penetrating a dielectric and a planar metallized (or semiconductor) anode, and extending from a planar semiconductor cathode, where the microcavity contains a conductive filler, such as gas or vapor, and the filler is electrically contacted by the semiconductor cathode.
Still another object of the present invention is to provide an improved microdischarge lamp including a microcavity in a silicon substrate (or silicon film on an insulating substrate such as glass) which contains a conductive filler, the filler being electrically contacted by one or more semiconductor electrodes formed in the silicon, wherein the lamp is operable as a hollow cathode discharge at a pd product (pressure.times.diameter) exceeding approximately 20 Torr-mm, depending on the selected ratio of the cavity length to the cavity aperture.
Still another object of the present invention is to provide an improved microdischarge device having a thin film, multilayered structure whereby the optical radiation from a single microdischarge or an array of microdischarges can be coupled into a planar optical waveguide.
Still another object of the present invention is to provide improved microdischarge arrays which can be locked in phase for providing optical radiation from an ensemble of emitters having well-defined characteristics.
Still another object of the present invention is to provide an improved array of microdischarges in which the microcavity extends through the substrate and electrodes are fabricated on opposite sides of the substrate, allowing gases or vapors to flow through the microdischarge cavities, in which the gases can be decomposed into a less hazardous form or converted into a more useful species.